Inbreeding depression is the reduced biological fitness in a given
population as a result of inbreeding, or breeding of related
Population biological fitness refers to an organism's
ability to survive and perpetuate its genetic material. Inbreeding
depression is often the result of a population bottleneck. In general,
the higher the genetic variation or gene pool within a breeding
population, the less likely it is to suffer from inbreeding
Inbreeding depression seems to be present in most groups of organisms,
but varies across mating systems.
Hermaphroditic species often exhibit
lower degrees of inbreeding depression than outcrossing species, as
repeated generations of selfing is thought to purge deleterious
alleles from populations. For example, the outcrossing nematode
Caenorhabditis remanei has been demonstrated to suffer
severely from inbreeding depression, unlike its hermaphroditic
relative C. elegans, which experiences outbreeding depression.
2 Natural selection
4 In humans
5 Factors reducing inbreeding depression
5.1 Purging selection
5.3 Selection for heterozygosity
6 See also
8 External links
Example of inbreeding depression
Inbreeding (i.e., breeding between closely related individuals)
results in more recessive traits manifesting themselves, as the
genomes of pair-mates are more similar.
Recessive traits can only
occur in an offspring if present in both parents' genomes. The more
genetically similar the parents are, the more often recessive traits
appear in their offspring. Consequently, the more closely related the
breeding pair is, the more homozygous, deleterious genes the offspring
may have, resulting in very unfit individuals. For alleles that confer
an advantage in the heterozygous and/or homozygous-dominant state, the
fitness of the homozygous-recessive state may even be zero (meaning
sterile or unviable offspring).
An example of inbreeding depression is shown to the right. In this
case, a is the recessive allele which has negative effects. In order
for the a phenotype to become active, the gene must end up as
homozygous aa because in the geneotype Aa, the A takes dominance over
the a and the a does not have any effect. Due to their reduced
phenotypic expression and their consequent reduced selection,
recessive genes are, more often than not, detrimental phenotypes by
causing the organism to be less fit to its natural environment.
Another mechanism responsible for inbreeding depression is the fitness
advantage of heterozygosity, which is known as overdominance. This can
lead to reduced fitness of a population with many homozygous
genotypes, even if they are not deleterious or recessive. Here, even
the dominant alleles result in reduced fitness if present homozygously
(see also hybrid vigour).
Currently, it is not known which of the two mechanisms is more
prevalent in nature. For practical applications, e.g. in livestock
breeding, the former is thought to be more significant – it may
yield completely unviable offspring (meaning outright failure of a
pedigree), while the latter can only result in relatively reduced
Natural selection cannot effectively remove all deleterious recessive
genes from a population for several reasons. First, deleterious genes
arise constantly through mutation within a population. Second, in a
population where inbreeding occurs frequently, most offspring will
have some deleterious traits, so few will be more fit for survival
than the others. Different deleterious traits are extremely unlikely
to equally affect reproduction – an especially disadvantageous
recessive trait expressed in a homozygous recessive individual is
likely to eliminate itself, naturally limiting the expression of its
phenotype. Third, recessive deleterious alleles will be "masked" by
heterozygosity, and so in a dominant-recessive trait, heterozygotes
will not be selected against.
When recessive deleterious alleles occur in the heterozygous state,
where their potentially deleterious expression is masked by the
corresponding wild-type allele, this masking phenomenon is referred to
as complementation (see complementation (genetics)).
In general, sexual reproduction in eukaryotes has two fundamental
aspects: recombination during meiosis, and outcrossing. It has been
proposed that these two aspects have two natural selective advantages
respectively. A proposed adaptive advantage of meiosis is that it
facilitates recombinational repair of DNA damages that are otherwise
difficult to repair (see DNA repair as the adaptive advantage of
meiosis). A proposed adaptive advantage of outcrossing is
complementation, which is the masking of deleterious recessive
alleles (see hybrid vigor or heterosis). The selective advantage
of complementation may largely account for the general avoidance of
inbreeding (see kin recognition).
Introducing alleles from a different population can reverse inbreeding
depression. Different populations of the same species have different
deleterious traits, and therefore their cross breeding will not result
in homozygosity at most loci in the offspring. This is known as
outbreeding enhancement, practiced by conservation managers and zoo
captive breeders to prevent homozygosity.
However, intermixing two different populations can give rise to unfit
polygenic traits in outbreeding depression (i.e. yielding offspring
which lack the genetic adaptations to specific environmental
conditions). These, then, will have a lowered fitness than pure-bred
individuals of a particular subspecies that has adapted to its local
The biological effects of inbreeding depression in humans are largely
obscured by socioeconomic and cultural influences on reproductive
behavior. Studies in human populations have shown that age at
marriage, duration of marriage, illiteracy, contraceptive use, and
reproductive compensation are the major determinants of apparent
fertility, even amongst populations with a high proportion of
consanguinous unions. However, several small effects on increased
mortality, longer inter-birth intervals and reduced overall
productivity have been noted in certain isolated populations.
Charles Darwin was one of the first scientists to demonstrate the
effects of inbreeding depression, through numerous experiments on
plants. Darwin's wife, Emma, was his first cousin, and he was
concerned about the impact of inbreeding on his ten children, three of
whom died at age ten or younger; three others had childless long-term
Factors reducing inbreeding depression
Whilst inbreeding depression has been found to occur in almost all
sufficiently studied species, some taxa, most notably some
angiosperms, appear to suffer lower fitness costs than others in
inbred populations. Three mechanisms appear to be responsible for
this: purging, differences in ploidy, and selection for
heterozygosity. It must be cautioned that some studies failing to
show an absence of inbreeding depression in certain species can arise
from small sample sizes or where the supposedly outbred control group
is already suffering inbreeding depression, which frequently occurs in
populations that have undergone a recent bottleneck, such as those of
the naked mole rat.
Purging selection occurs where the phenotypes of deleterious recessive
alleles are exposed through inbreeding, and thus can be selected
against. This can lead to such detrimental mutations being removed
from the population, and has been demonstrated to occur rapidly where
the recessive alleles have a lethal effect. The efficiency of
purging will depend on the relationship between the magnitude of the
deleterious effect that is unmasked in the homozygotes and the
importance of genetic drift, so that purging is weaker for non-lethal
than for recessive lethal alleles. For very small populations,
drift has a strong influence, which can cause the fixation of
sublethal alleles under weak selection. The fixation of a single
allele for a specific gene can also reduce fitness where heterozygote
advantage was previously present (i.e., where heterozygous individuals
have higher fitness than homozygotes of either allele), although this
phenomenon seems to make a usually small contribution to inbreeding
depression. Although naturally occurring, purging can be important for
population survival, deliberately attempting to purge deleterious
mutations from a population is not generally recommended as a
technique to improve the fitness of captive bred animals.
Many angiosperms (flowering plants) can self-fertilise for several
generations and suffer little from inbreeding depression. This is very
useful for species which disperse widely and can therefore find
themselves growing in a novel environment with no conspecifics
Polyploidy (having more than two paired sets of each
chromosome), which is prevalent in angiosperms, ferns and a select few
animal taxa, accounts for this. By having several copies of a
chromosome, as opposed to two, homozygosity is less likely to occur in
inbred offspring. This means that recessive deleterious alleles are
not expressed as frequently as with many copies of a chromosome; it is
more likely that at least one will contain a functional allele.
Selection for heterozygosity
Inbreeding depression has also been found to occur more gradually than
predicted in some wild populations, such as in the highly inbred
population of Scandinavian wolves. This appears to be due to a
selection pressure for more heterozygous individuals, which generally
are in better condition and so are more likely to become one of the
few animals to breed and produce offspring.
Minimum viable population
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